That's another great point, because what the Eldar and Harlequins may consider the most major players may not be what humans consider. It's their re-telling.
There are clearly people that are fundamentally important even in their version of history:
Then there are those the Eldar are disproportionately interested in, because it relates to them:
Fulgrim, because he and his Legion are most closely aligned with their adversary, Slaanesh
Ahriman, like you mentioned, because he's always trying to get in their Black Library
Furthermore, just as we have to consider that they may not know things that humans do, we have to consider what they do know that humans don't, like:
The Lost Primarchs, II and XI
Yeah, I normally wouldn't even mention the Lost Primarchs, because it just opens up a Pandora's Box. However, with your idea, having them represented by Harlequins...who act as a buffer or foil between us...we don't really open that box; instead, the Harlequins do, and they don't tell us what's inside, they only tell each other. All we would see is that they existed in some way, but that's already a known issue. Moreover, the Eldar were always said to be keepers of forbidden knowledge, but over the editions more and more information about them has been revealed, to the point they lost that quality, that mystique. Having them represented by Harlequins, perhaps something as simple as just having them marked as II and XI, would be really impactful.
Sidenote, I'm thinking of making a Death Jester character that represents the Mechanicum/Mechanicus. They already have the half-skull motif, and a big gun, plus in Master of Mankind they had a strange role in the webway.